I found this site that has a calculator to help you find out what gearing you should use. I know he has a jeep but I think it works for us also. I noticed that changing to 32.6" tires today really puts my 3.55 gears to the test.

http://www.yuccaman.com/jeep/speedo.html

 

There are many sights out there like that one.

My dad's 04 F150 w/ 3.73s had stock 32" tires, and he moved to 34" tires giveing him an affective gear ratio of 3.54.

And for the people that put 37" tires on a truck that came original with 31s and 3.55 gears and don't regear, they really feel a power loss in acceleration and a loss in mileage, b/c the affective gear ratio would be about 2.80. And they just can't seem to understand why they're truck is so weak after big tires either.....